A number of methods exist for estimating bandwidth, and can be categorised in the following three groups:
“Max throughput”—a channel is loaded with data until increased one-way delay, packet round-trip time RTT, and/or loss is observed. The bandwidth is the maximum load that goes through without problems.
“Relative delay”—packets of different sizes are sent and their individual RTTs/delays are measured. Assuming that one-way transmission times equal packet sizes divided by bandwidth, the bandwidth can be estimated from the slope of the observed (packet size, RTT/delay) graph. The packets must be transmitted with a large interval to avoid network queuing, which would otherwise obscure the measurements. Typically, RTTs are employed instead of one-way delays because of the unknown clock offset between transmitter and receiver. In a modified version, two packets are sent back-to-back meaning that the last one will always be queued behind the first one, and the bandwidth can be determined from the difference in the two packets' RTTs/delays. For a surveillance of such methods, see e.g. http://vbn.aau.dk/fbspretrieve/6189553/Available_Bandwidth_Estimation. pdf.